Synthetic aperture radar (SAR) is primarily used as a radar mapping technique for generating high resolution images of a desired imaging area such as a desired geographic area. Synthetic aperture radar imagery is well known and is often used for terrain mapping, particularly in remote areas accessible only from the air. Synthetic aperture radar imaging techniques are often utilized in both aircraft based and space based systems for this purpose.
It is well known in the radar art that the resolution of a conventional radar system can be enhanced by increasing the effective antenna length. In order to increase radar image resolution, linear antenna arrays have been utilized having a plurality of antenna elements functioning together to increase an effective antenna length. Synthetic aperture radar, however, typically utilizes a single antenna element which traverses a path extending across the area to be imaged. By traversing the area to be imaged with the radar antenna, the antenna is synthetically lengthened to increase the effective antenna length enabling increasing image detail to be resolved.
As will be explained further hereinbelow with reference to the preferred embodiments, synthetic aperture radar determines the range to a target object from the time delay between the generation of an interrogation pulse and receipt of a position indicating reflection. A typical system may utilize linear FM chirped pulses having a linearly varying carrier frequency. In such a case the reflection delay which is indicative of distance is determined by the frequency of the received radar reflection upon proper demodulation.
Azimuth or cross range is more difficult to determine accurately, however. The azimuth resolution of a conventional antenna is limited to the transmitted beam width. However, in synthetic aperture radar the phase shift between successive position-indicating reflections can accurately establish the azimuth or cross-range position of received reflections. In order to accurately image a desired area, it is important for such a synthetic aperture system to accurately know the path which the antenna travels. Accordingly, as in the FIG. 1 embodiment of the present invention, an aircraft's inertial navigation system is often utilized to develop an accurate aircraft track. This is sufficient for accurate and continuous determination of the position of the radar antenna and also allows an accurate determination of range to the target of interest, as such range is measured through measurement of the time delay between transmission of a radar interrogation pulse and receipt of the reflection therefrom. Only minor range errors therefore occur due to errors in the position of the antenna. However, antenna track errors are considerably more serious in their effect on azimuth, or crossrange measurements due to the manner in which these measurements are made.
In synthetic aperture radar, the target area is typically interrogated by sequentially transmitted radar pulses. Azimuth or cross-range is resolved by coherently integrating echo energy reflected from the target area as the aircraft carrying the radar travels past the target area. In sequential samples of the target area from an aircraft following a linear track, the reflections from a single target element differs in phase from each other due to the change in distance from the reflected object caused by traversal of the mapping aircraft. These changes in phase are indicative of azimuthal position of the target with respect to the mapping aircraft's track. Unfortunately, however, small uncompensated phase changes create significant azimuthal ambiguities. These so-called phase errors result from various factors including, in particular, uncompensated change in the target's range from the mapping aircraft's track. Space borne synthetic aperture radar systems also encounter arbitrary phase error due to ionospheric disturbances.
The problem of the above mentioned phase errors adversely affecting synthetic aperture radar imagery has been addressed by others in the art. Several prominent techniques have been developed. One such technique attempts to produce a fixed order approximation of the phase error by performing an nth order orthogonal polynomial fit to the known phase error function. Such a technique is used by the known Map-Drift algorithm which makes use of two properties of synthetic aperture radar. The Map-Drift algorithm displays a first property in that when the synthetic aperture is divided into a number of sub-apertures, each sub-aperture will develop an image of the same terrain except for a reduction in resolution. A second property is that a linear phase error across the aperture shifts the entire scene in the azimuthal direction. Because of these factors, the Map-Drift technique divides the aperture into several sub-apertures and determines the degree of shifting of the images with respect to each other. The degree of shift between respective images produced from respective sub-apertures is proportional to the quadratic phase error therebetween and from a knowledge of these sub-aperture phase errors the Map-Drift system develops an n-th order polynomial to approximate the total phase error. Thus, the phase error can be determined by relative image displacements.
Map-Drift is a parametric technique and is described, for example, in the publication entitled "Some Methods For Reducing Propagation-induced Phase Errors In Coherent Imaging Systems, I. Formalism." by Warren C. Brown and Dennis C. Ghiglia, published June 1988 in the Journal of the Optical Society of America, Vol. 5. In particular, on page 927, this known method is described. An nth order polynomial is constructed from this phase error. Due to the constraints of such a Map-Drift technique, the algorithm is limited to the development of about a fifth-order polynomial to compensate for existing phase error. Accordingly, Map-Drift cannot accurately compensate for higher order phase errors which degrade azimuthal performance of synthetic aperture radar imaging.
Another technique utilized to compensate for phase errors in synthetic aperture radar is the so-called prominent-point technique. The Prominent-Point algorithm exploits a fundamental property of linear systems. A linear system is completely described by its impulse or point response. When the synthetic aperture image includes a dominant, point signature which can be isolated from the background, the degree of phase error degradation of the image can be estimated from the degree of degradation of the image at this point. By filtering the image with the inverse of the error, the phase error can be substantially eliminated. However, the Prominent-Point compensation fails when the degraded scene does not contain a strong isolated point signature or when the point signature cannot be separated from other interfering sources. Accordingly, Prominent-Point compensation cannot readily operate without strong point reflections.
While the above mentioned phase error problems occur in synthetic aperture radar imaging, other types of synthetic aperture imaging may encounter similar problems. For example, synthetic aperture sonar can encounter the phase errors discussed above. These errors can be readily and accurately corrected by the teachings of the present invention.
Subsequent to the discovery of the present invention, the applicants have discovered that the present invention utilizes processing techniques similar to the process techniques previously used for the removal of "blur" in long exposure optical images produced by ground-based optical telescopes due to atmospheric fluctuations. This so-called stellar speckle processing utilizes mathematical techniques similar to those developed by the inventors. However, substantial differences exist between such stellar processing techniques and the concepts of the present invention. In the known stellar processing techniques, in order to resolve the star images of interest, a number of short exposures of the object of interest are produced with each exposure "freezing" the atmosphere so that the image is not blurred. However, due to the atmospheric fluctuations, each exposure image does not individually represent the object of interest. Digital processing of hundreds of speckle images can mitigate the effects of the change in atmosphere and produce a diffraction limited image of the object common to each speckle image. In such stellar processing the phase error inducing atmospheric fluctuations vary over time while the image remains unchanged. In contrast, in synthetic aperture imaging the phase error for each image is time invariant while the imaged target varies. Thus, the phase error in a synthetic aperture radar image is redundant in the degraded image. Each target portion in the image is degraded by the same phase error and this redundancy may be used to extract the phase error information from the image. Further, synthetic aperture radar imaging offers the advantage that the complex imagery may be stored, thus allowing phase correction iterations which cannot be performed in the so-called speckle processing used for optical correction of blur in the viewing of stellar images. In contrast, in speckle imaging, it is the image scene which is redundant in the multiple speckle images with the phase error varying throughout. This leads to substantial distinctions in the imaging and its correction as utilized in these two systems.